Covalent attachment of hydrophilic polymers to molecules having pharmaceutically useful properties is of considerable utility for drug delivery. There is a growing list of polymers whose conjugates have entered clinical trials. Among them are conjugates of polyethylene glycol, abbreviated as “PEG,” [Greenwald et al. (2003) Effective drug delivery by PEGylated drug conjugates. Adv. Drug Delivery Rev. 55:217-250; Harris et al. (2003) Effect of PEGylation on Pharmaceuticals. Nat. Rev. Drug Discovery 2:214-221)], hydroxyethylcellulose, abbreviated as “HES,” (WO 2006/050959), poly(L-glutamic acid) [Li (2002) Poly(L-glutamic acid)-anticancer drug conjugates. Adv. Drug Delivery Rev. 54: 695-713]. PEG conjugates have been remarkably successful as several are marketed drugs (e.g. CIMZIA®, NEULASTA®, MACUGEN®, SOMAVERT®, PEGASYS®, and PEG-INTRON®). PEG is a polymer that possesses many beneficial properties. For instance, PEG is soluble in water and in many organic solvents, is non-toxic and non-immunogenic, and when attached to a surface, PEG provides a biocompatible, protective coating. Common applications or uses of PEG include (i) covalent attachment to proteins to, for example, extend plasma half-life and reduce clearance through the kidney, (ii) attachment to surfaces such as in arterial replacements, blood contacting devices, and biosensors, (iii) use as a soluble carrier for biopolymer synthesis, and (iv) use as a reagent in the preparation of hydrogels. The other commonly used hydrophilic polymers claim similar properties and potential uses.
In many if not all of the uses noted above, it is necessary to first activate the hydrophilic polymer by converting its active terminus, e.g., a hydroxyl group in the case of a PEG, to a functional group capable of readily reacting with a functional group found within a desired target molecule or surface, such as a functional group found on the surface of a protein. For proteins, typical functional groups include functional groups associated with the side chains of lysine, cysteine, histidine, arginine, aspartic acid, glutamic acid, serine, threonine, and tyrosine, as well as the N-terminal amino functional group and the C-terminal carboxylic acid functional group. Other nontoxic biocompatible hydrophilic polymers may be substituted and are generally acceptable alternatives, with modest changes based on the specific functional groups that are available for use in polymer modification and ultimately conjugation.
Using PEG as representative of the class, PEG used as a starting material for most PEG activation reactions is typically an end-capped PEG. An end-capped PEG is one where one or more of the hydroxyl groups, typically located at a terminus of the polymer, is converted into a non-reactive group, such as a methoxy, ethoxy, or benzyloxy group. Most commonly used is methoxyPEG, abbreviated as mPEG. End-capped PEGs such as mPEG are generally preferred, since such end-capped PEGs are typically more resistant to cross-linking and aggregation. The structures of two commonly employed end-capped PEG alcohols, mPEG and monobenzyl PEG (otherwise known as bPEG), are shown below,
wherein n typically ranges from about 10 to about 2,000.
In one specific example of a polymer reagent for use in drug delivery, U.S. Pat. No. 6,436,386 describes PEG-based, hydroxyapatite-targeting polymers that can be used to selectively target bone surfaces within a patient for delivery of therapeutic agents to the bone site. In this manner, the polymeric reagent provides both targeted delivery of the active portion of the molecule to the tissue of interest and increased circulation time.
Despite many successes, conjugation of a polymer to an active agent is often challenging. For example, it is known that attaching a relatively long poly(ethylene glycol) molecule to an active agent typically imparts greater water solubility than attaching a shorter poly(ethylene glycol) molecule. One of the drawbacks of some conjugates bearing polymer moieties, however, is the possibility that such conjugates may be substantially inactive in vivo. It has been hypothesized that these conjugates are substantially inactive due to the length of the polymer chain, which effectively “wraps” itself around the entire active agent, thereby limiting access to ligands required for pharmacologic activity.
As a result, there is an ongoing need in the art for polymer reagents suitable for conjugation to drug moieties for drug delivery applications, particularly polymer reagents that have the molecular weight necessary to provide a for a conjugate that has the desirable in vivo circulation time, but which also exhibits timely clearance from the body. It would be particularly beneficial for such polymer reagents to also provide the ability to target a particular site of the body, such as hydroxyapatite surfaces. The present invention addresses this and other needs in the art.